1. FIELD OF THE INVENTION
This invention pertains to variable resistor position sensors.
2. DESCRIPTION OF THE RELATED ART
Many internal combustion engines use a throttle valve to control the amount of air entering the engine. The throttle valve is also commonly called a butterfly or throttle flap. Throttle valves are used in gasoline, diesel and other alternatively fueled vehicles. The throttle valve may be opened to provide unimpeded air intake through a throttle body. Alternatively, the throttle valve may be closed to greatly restrict the passage of air. By controlling the amount of air that reaches the combustion chamber, the throttle valve forms part of the primary engine speed control. The throttle valve may be mechanically linked to the accelerator pedal or, in some instances, linked through a combination of electrical and mechanical interconnections.
There are many efforts to improve the efficiency of internal combustion engines and similarly to reduce the emissions, or pollutants, that are produced by these engines. A vital part of better efficiency and reduced emissions is the electronic control circuitry used with the engines. The electronic circuitry monitors various engine parameters and provides feedback or controls to the engine. The feedback may be a signal which in some way improves efficiency or reduces emissions. The signal may, for example, be used to control the amount of fuel injected into the engine or the timing of ignition sparks.
A potentiometer is often used to sense the position of the throttle valve. This potentiometer is in some ways similar to the volume controls used in radio and television receivers. A voltage is applied across two extreme ends of a resistor. An intermediate tap is provided between the two extremes of the resistor. The tap is mechanically linked to the device which is to be sensed, and the position of the device is determined by the voltage at the intermediate tap.
There are several stringent requirements placed upon a throttle position sensor that make it different from a volume control. Since the throttle valve is used to control air intake and thereby represent a demand for power, binding of the throttle shaft in an open throttle position could result in life threatening situations. Safety and reliability are essential in automotive applications.
The automotive environmental requirements are also different from a radio or television receiver. The throttle position sensor must reside against the throttle body. Temperatures might, for example, range from -55 to +150.degree. degrees celsius. Further, the device may be exposed to a number of solvents, road spray, and other adverse conditions associated with engine compartment environments. These requirements diverge greatly from the typical volume control.
Examples of conventional throttle position sensors include U.S. Pat. 4,430,634 by Hufford et al and assigned to the present assignee and also U.S. Pat. No. 4,355,293 by Driscoll and 5,133,321 by Hering et al, incorporated herein by reference. Other examples may be found in U.S. Pat. Nos. 4,616,504, 4,621,250, 4,688,420, 4,703,649, 4,715,220, 4,719,795, 4,743,882, 4,812,803, 4,933,661, 5,133,321, and Japanese Kokai 58-70104, also incorporated herein by reference.
In the prior art, a lever such as shown in U.S. Pat. Nos. 4,355,293 and 4,430,634 or special drives such as shown in U.S. Pat. No. 4,616,504 were used. These drives ensure that, even in the event of malfunction, the throttle sensor will not retain the throttle valve in an acceleration position, but instead will allow the throttle valve to return to an idle. Engagement between the sensor and the throttle shaft has then necessitated the use of a return spring so that as the throttle shaft returns to idle position, the throttle position a sensor also returns and tracks the position of the throttle valve.
The throttle position sensor in the nor art is a freestanding, rather self-contained device. In addition to the return spring, a well-sealed package including the associated bearings is typically provided. Significant effort was directed at designing a package that was sealed against the adverse chemicals and moisture that might otherwise damage the sensor.
Inclusion of the spring and bearings into this sealed package has drawbacks. The use of springs requires a fairly robust design. Springs and bearings add expense to the device and increase the cost and hazards of assembly. Additionally, any wear debris that may result from the spring or bearings may be detrimental to the operation of the position sensor.
Other prior art sensors incorporate the sensor directly into the throttle body. Exemplary of this concept are U.S. Pat. Nos. 4,649,367, 4,672,356, 4,693,111, 4,718,272, 4,827,884, 4,866,981 and 5,070,728 incorporated herein by reference. This concept offers advantage in simplicity. However, there is little control over the element contactor interface, which has been determined to be very important for the life of the unit.
Variations in contact pressure, contact orientation, lube and other similar factors all impact the performance of the device. Further, field replacement is important for service repair, and the service replacement should be of the same quality as the original device. These throttle body incorporated sensors do not have the precise control over lube thickness and composition, protection of vital components while shelved awaiting installation, and control over contactor and element relationships that are desirable features.
The shape of the contactor structure is, for obvious reasons, critical to the performance of the device. Where contactor rakes are used, a bent rake may reduce the life of the device to less than one hundredth the normal life. Yet, in those devices that mount into the throttle wall, the contactor will be exposed during shipment of service parts and will be handled to an undesirable degree during installation.
With electronics becoming more prevalent, the ability to sense various engine functions and also in some instances non-engine or indirect engine functions is more desirable. The present invention seeks to overcome the limitations of the prior art sensors and offer a throttle position sensor that delivers unmatched performance without compromise. Further, while the preferred embodiment is certainly throttle position sensing, the inventive features are applicable to position sensors in other applications, including but not limited to accelerator pedal position sensing, machine and industrial robot position sensing, and other applications for potentiometric devices of high quality and reliability.